In the monolithic integrated circuit technology, capacitors are a common element. Large numbers of capacitors, each associated with a Field Effect Transistor (FET), are required in Dynamic Random Access Memory (DRAM) chips. As the requirement for additional memory capacity has increased, so has the requirement for packing the capacitors at higher and higher densities. The original planar capacitor design, which creates a capacitor on the chip surface, occupies too much of the chip surface per capacitor. Capacitor designs which form the capacitors in trenches in the silicon wafer permit higher capacitor densities and are the trend for the future.
Trench capacitors require, however, deep and narrow trenches having a very high aspect ratio--often more than 40:1. Typically, trench capacitors are formed in the trench by depositing a dielectric layer on the trench walls and filling the trench with a doped polysilicon layer to form the upper capacitor plate. A doped silicon trench wall area forms the lower capacitor plate.
Trenches having an aspect ratio greater than 4:1 are generally considered to have a high aspect ratio. As deep trench aspect ratios exceed about 10:1, filling the trench becomes more and more difficult. The top portion of the trench tends to receive more deposition, thereby blocking diffusion of reactants to the bottom portion of the trench. Often, the result is the creation of voids within the fill. Such voids increase buried plate series resistance. Once the trench is filled with silicon, the voids do not disappear with additional processing. In fact, the voids may get larger as the wafers go through the various anneal cycles, especially for amorphous silicon fill.
In addition to trenches having a substantially rectangular cross section, trench structures having a bottle-shaped cross section with a narrower opening at the top than at the bottom of the trench are also used. It is theoretically impossible to completely fill such bottle-shaped trenches with a single deposition step.
To assure more complete fill of trench structures, a lower deposition temperature has been used at the expense of reduced deposition rates and longer processing times. Multiple deposition steps may be necessary, especially for bottle-shaped trenches, including etching steps between depositions to re-open the top of the trench and allow penetration of additional fill material into the trench. Such multiple steps complicate the processing sequence.
Thus, a need exists for a more practical method to fill such deep or odd-shaped trenches.